As digital circuitry operates, the circuitry generates impulses of current drawn from a regulated power supply (Vdd). In general, digital circuitry performs a task or operation with each clock cycle, and the task or operation is completed by the digital circuitry before the next clock edge is received. Accordingly, when a digital circuit is actively processing data, the current drawn by the digital circuit tends to vary periodically with the clock signal. Such variations can appear as pulses or transient variations in the power supply current or can produce transient decreases in the power supply voltage. Assuming that the current drawn by the digital circuit follows a periodic pattern, such as pulses of a clock signal, the current drawn by the digital circuit can introduce a sawtooth waveform on the power supply current.
Some of the energy from such current variations can reach a power supply input pin of the circuit (due to finite reverse power supply rejection ratio (RPSRR)). Such energy at the power supply input pin can radiate spectral energy, producing radio frequency interference that can interfere with nearby receiver circuitry.
Conventionally, circuitry has been implemented to control the power supply circuitry to produce a power supply current having reduced AC components. In one example, power supply regulators are designed so that variations in the load current from the digital circuitry are only minimally coupled back to current variations at the power supply input pin and to the supply current flowing external to the integrated circuit. While such regulators reduce external supply current variations, the digital load current may still vary.